Django’s URL handling is based on a mapping of regex patterns
(representing the URLs) to callable views, and Django’s own processing
consists of matching a requested URL against those patterns to
determine the appropriate view to invoke.

Django also provides a convenience function –
reverse() – which performs this process
in the opposite direction. The reverse() function takes
information about a view and returns a URL which would invoke that
view. Use of reverse() is encouraged for application developers,
as the output of reverse() is always based on the current URL
patterns, meaning developers do not need to change other code when
making changes to URLs.

One argument signature for reverse() is to pass a dotted Python
path to the desired view. In this situation, Django will import the
module indicated by that dotted path as part of generating the
resulting URL. If such a module has import-time side effects, those
side effects will occur.

Thus it is possible for an attacker to cause unexpected code
execution, given the following conditions:

One or more views are present which construct a URL based on user
input (commonly, a “next” parameter in a querystring indicating
where to redirect upon successful completion of an action).

One or more modules are known to an attacker to exist on the
server’s Python import path, which perform code execution with side
effects on importing.

To remedy this, reverse() will now only accept and import dotted
paths based on the view-containing modules listed in the project’s URL
pattern configuration, so as to ensure that only modules
the developer intended to be imported in this fashion can or will be imported.

The caching framework includes an option to cache responses to
anonymous (i.e., unauthenticated) clients.

When the first anonymous request to a given page is by a client which
did not have a CSRF cookie, the cache framework will also cache the
CSRF cookie and serve the same nonce to other anonymous clients who
do not have a CSRF cookie. This can allow an attacker to obtain a
valid CSRF cookie value and perform attacks which bypass the check for
the cookie.

To remedy this, the caching framework will no longer cache such
responses. The heuristic for this will be:

If the incoming request did not submit any cookies, and

If the response did send one or more cookies, and

If the Vary:Cookie header is set on the response, then the
response will not be cached.

The MySQL database is known to “typecast” on certain queries; for
example, when querying a table which contains string values, but using
a query which filters based on an integer value, MySQL will first
silently coerce the strings to integers and return a result based on that.

If a query is performed without first converting values to the
appropriate type, this can produce unexpected results, similar to what
would occur if the query itself had been manipulated.

Django’s model field classes are aware of their own types and most
such classes perform explicit conversion of query arguments to the
correct database-level type before querying. However, three model
field classes did not correctly convert their arguments:

These three fields have been updated to convert their arguments to the
correct types before querying.

Additionally, developers of custom model fields are now warned via
documentation to ensure their custom field classes will perform
appropriate type conversions, and users of the raw() and extra() query methods – which allow the
developer to supply raw SQL or SQL fragments – will be advised to ensure they
perform appropriate manual type conversions prior to executing queries.

Historically, queries that use
select_for_update() could be
executed in autocommit mode, outside of a transaction. Before Django
1.6, Django’s automatic transactions mode allowed this to be used to
lock records until the next write operation. Django 1.6 introduced
database-level autocommit; since then, execution in such a context
voids the effect of select_for_update(). It is, therefore, assumed
now to be an error and raises an exception.

This change was made because such errors can be caused by including an
app which expects global transactions (e.g. ATOMIC_REQUESTS set to True), or Django’s old autocommit
behavior, in a project which runs without them; and further, such
errors may manifest as data-corruption bugs.

This change may cause test failures if you use select_for_update()
in a test class which is a subclass of
TransactionTestCase rather than
TestCase.